The invention relates to a device for generating a vacuum for a vacuum cementing device. The invention also relates to a method for generating a negative pressure in a vacuum cementing system.
Conventional PMMA bone cements have been known for decades and are based on the ground-breaking work of Sir Charnley (Charnley, J., “Anchorage of the Femoral Head Prosthesis of the Shaft of the Femur,” J. Bone Joint Surg. 42: 28-30 (1960)). The basic structure of PMMA bone cements has remained the same ever since. PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component consists of one or more polymers made by polymerization, preferably suspension polymerization, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, a radio-opacifier, and the initiator, dibenzoylperoxide. Mixing the powder component and the monomer component, a dough that can be shaped plastically is generated by swelling of the polymers of the powder component in the methylmethacrylate. When the monomer component and the powder component are being mixed, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide which disintegrates while forming radicals. The radicals thus formed trigger the radical polymerization of the methylmethacrylate. Upon advancing polymerization of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies and thus is cured.
Polymethylmethacrylate bone cements can be mixed by mixing the cement powder and the monomer liquid in suitable mixing beakers with the aid of spatulas. This procedure is disadvantageous in that air inclusions may be present in the cement dough thus formed and cause destabilization of the bone cement later on. For this reason, it is preferred to mix bone cement powder and monomer liquid in vacuum mixing systems, since mixing in a vacuum removes air inclusions from the cement dough to a large extent and thus achieves optimal cement quality.
Bone cements mixed in a vacuum have clearly reduced porosity and thus show improved mechanical properties. A large number of vacuum cementing systems have been disclosed of which the following shall be listed for exemplary purposes: U.S. Pat. Nos. 5,624,184; 4,671,263; 4,973,168; 5,100,241; 5,586,821; and 5,344,232; International patent application publication Nos. WO99/67015 and WO94/26403; European patent application publications EP 1 020 167 A1; EP 1 016 452 A1; EP 0 692 229 A1; and EP 1 005 901 A1; and German published patent application DE 36 40 279 A1.
In the vacuum cementing systems that are currently common in the market, the requisite vacuum is generated by vacuum pumps driven by non-sterile compressed air. These hose systems are heavy and always associated with a risk of accidents since they are routed on the floor. Moreover, it is also feasible to provide vacuum in the operating theatre by central vacuum line systems. Since long hoses are used in this case, this also results in problems related to the risk of accidents and poor handling properties. Moreover, a vacuum needs to be generated at the site of the surgery or in its immediate vicinity, which means that a supply of compressed air and/or electrical power needs to be available. This is not always feasible if a surgery (operating room) is to be performed in less developed regions, at an accident site, in makeshift emergency operating theatres, in tents, or in a field hospital.